Electronic switches, oscillators, and amplifiers are often formed using a field effect transistor (FET). The FET has three basic parts; a gate comprising a single metal pad, a thin insulating layer underneath the gate, and a channel, made from a semiconducting material, underneath the insulating layer. The application of a voltage to the gate results in an electric field that, depending on its sign, either accumulates carriers at the semiconductor-insulator interface or depletes them from this region. The former process increases the electrical conductivity of the semiconducting layer; the latter decreases it. These effects enable the FET to act as an amplifier or switch and enable it to be used as a component in an oscillator source producing oscillatory electrical currents or voltages.
The body of the FET is conventionally a doped semiconductor. The doping defines a necessary reference level against which the electric field provided by the gate acts. Technically, this is known as “pinning the Fermi energy of the semiconductor”. Because a major change in the carrier density at the semiconductor-insulator interface is required for operation, the FET suffers from time lags due to the capacitance associated with the dopants and with moving the charge carriers within the thickness of the device. The operating speed of the FET is limited by associated effects: the carrier mobility and the capacitance of the depletion layer.
Another, less closely-related device is the bipolar transistor, which also acts as a switch, amplifier, or oscillator. In addition to being speed-limited by carrier mobility, it also relies on minority carriers and is limited in speed by carrier recombination times.
A recent improvement in FET technology is described in U.S. Pat. No. 6,724,056, issued Apr. 20, 2004, where the conventional dielectric material between the gate and the semiconductor material is formed from a charge or spin density wave (CDW or SDW, respectively, material). This device used the high value of the real part of the dielectric constant for these materials, i.e., the insulating value, to make the gate structure ultra-sensitive.
In accordance with the present invention, the semiconductor of a conventional FET is replaced by a charge or spin density wave (CDW or SDW, respectively) material, and, secondly, the continuous gate of a FET is replaced by periodic electrode structure or some other means of applying a periodically-varying electric field. In a CDW or SDW material, interactions between the conduction electrons result in the material spontaneously changing from a metal (i.e. an electrical conductor) to a quantum coherent state (the density wave) that is an electrical insulator. A defining characteristic of the latter insulating state is a periodically-modulated charge—(in the case of a CDW) or spin—(in the case of an SDW) density. The periodic electrode structure enables this process to be controlled through the application of a matching periodic electrostatic field that either destroys the density wave, or enhances its formation.
This results in an electronic device, called herein a “quantum coherent switch” (QCS) that may exhibit electrical switching action, amplification, or oscillation. Because the density-wave (DW) state is a cooperative phenomenon, i.e., an effectively instantaneous, self-reinforcing transition driven by the mutual interactions between the conduction electrons, the speed of the device is not limited by carrier recombination times, carrier mobility or capacitative effects. Moreover, the QCS does not require the use of a conventional semiconductor or the addition of doping. The periodic electrostatic field can be applied by an insulated periodic metallic gate electrode structure or by a standing electromagnetic wave, for instance within a resonant cavity.
One advantage inherent in the elimination of doping is that the QCS may be made arbitrarily small; in a conventional FET, a relatively large volume of doped semiconductor is required to adequately pin the Fermi energy. Another advantage is that, by exploiting the intrinsically long phase correlation length of DWs, the energy density involved in switching is much smaller than with conventional semiconductor devices, such as FETs. The required signal voltages are at least an order of magnitude smaller. Further, because the DW substrate material is subjected to a spatially modulated electric field, as opposed to one that is uniform, the total change in carrier concentration induced in the DW material is zero. Hence, there are no capacitative effects associated with moving large quantities of charge around. The switching speed will instead be determined by the frequency corresponding to the energy gap of the DW, which may greatly exceed 1 THz.
Because DW formation is a bulk cooperative phenomenon, the system is able to undergo abrupt transitions from metallic to insulating behavior (or vice versa) upon being subjected to only very weak perturbations. Such perturbations can be in the form of an electrostatic potential, or additionally a change in temperature or magnetic field; the timescale for such transitions will be determined by the characteristic quantum-mechanical energy scale of the DW, which will typically fall in the 10 s-100 s of THz frequency range.
The theory of density wave materials is well known; see, e.g., R. E. Thorne, Charge Density Wave Conductors, Phys. Today, p. 42 (May 1996) and references cited therein. U.S. Pat. No. 4,636,737, issued Jan. 13, 1987, to Bhattacharya et al., describes a device using a density wave material, where the oscillating properties of a charge density wave conductor are used to form a demodulation device. U. S. Pat. No. 6,735,073, issued May 11, 2004, describes capacitive devices using the high dielectric values of DW materials to yield high capacitance devices. These devices do not use a periodic spatial electrostatic potential applied to the material surface to provide switching or amplification.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.